1. Field of the Invention
The present invention relates to a polymerization solution for electrolytic polymerization having a small environmental load, having excellent economic efficiency and providing a conductive polymer exhibiting excellent heat resistance. The present invention also relates to a conductive polymer film and a solid electrolytic capacitor obtained from the polymerization solution.
2. Description of the Related Art
A conductive polymer obtained from polymerization of monomer having a π-conjugated double bond such as thiophene, aniline and pyrrol is excellent in conductivity and suitable for various kinds of electrochemical purposes, and has been applied in a wide range of fields such as solid electrolytic capacitors, polymer batteries, antistatic films, indicating elements, sensors and electrode materials. For example, the conductive polymer is preferably used in a solid electrolytic capacitor that contains a positive electrode made of valve metal foil such as aluminum foil, tantalum foil and niobium foil that has an oxide film as a dielectric on the surface, and a conductive polymer film that is adjacent to the oxide film and acts as a real negative electrode.
A polymerization solution for electrolytic polymerization used for preparing the conductive polymer usually contains a monomer having a π-conjugated double bond, a supporting electrolyte and a solvent for dissolving them.
The solvent of the polymerization solution is appropriately selected considering the solubility of the monomer. For example, Patent Document 1 (JP 61-239617 A) discloses a method of preparing a conductive polymer film by electrolytic polymerization of aniline substituted by an alkyl group or an alkoxyl group, and in working examples, water is used as a solvent of a polymerization solution. Patent Document 2 (JP 1-313521 A) discloses a method of preparing a conductive polymer by electrolytic polymerization of 3,4-disubstituted thiophene including 3,4-ethylenedioxythiophene (3,4-ethylenedioxythiophene is hereinafter referred to as “EDOT” and poly(3,4-ethylenedioxythiophene) is hereinafter referred to as “PEDOT”), and in working examples, acetonitrile is used as a solvent of a polymerization solution. Also, Patent Document 3 (JP 3-18009 A) discloses a solid electrolytic capacitor that contains a conductive polymer film formed by electrolytic polymerization of non-substituted pyrrol, and in working examples, acetonitrile or water is used as a solvent of a polymerization solution.
The supporting electrolyte acting also as a dopant is appropriately selected considering its solubility in the solvent and the fact that the dopant is not easily detached from a polymer obtained, and is used with a concentration at which sufficient current can be obtained for electrolytic polymerization. A vast number of supporting electrolytes have been suggested so far. For example, Patent Document 4 (JP 2010-37466 A) summarizes in paragraph 0026 dopants emitted from supporting electrolytes conventionally suggested such as perchlorate ion, p-toluenesulfonate ion and borodisalicylate ion, and suggests a benzenesulfonic acid salt substituted by a branched alkyl group as a supporting electrolyte that provides higher heat resistance than conventional supporting electrolytes. Patent Document 5 (JP 2000-269087 A) discloses using an aqueous polymerization solution in which a thiophene derivative is emulsified by an alkylnaphthalenesulfonate surfactant to produce a solid electrolytic capacitor. Because of the bulkiness of an alkylnaphthalenesulfonate anion which is taken into the polymer layer as a dopant by electrolytic polymerization, de-doping is inhibited, and a solid electrolytic capacitor, which is stable under high temperature and high humidity, is obtained. Further, Non-Patent Document 1 (“Synthesis of High-Function Conductive Polymers (PPy, PEDOT, etc.), Development of Application Fields to Electronic and Energy Devices and Future Prospects” Proceedings, pp. 30-33) describes electrolytic polymerization of pyrrol, and sodium triisopropylnaphthalenesulfonate is used as a supporting electrolyte.
However, the use of organic solvents generally increase environmental burdens and is economically disadvantageous compared with the use of water. Moreover, many of the organic solvents are harmful to humans, and in the case of electrolytic polymerization using a combustible solvent, procedures need to be taken to prevent fire by electric spark. Therefore, it is preferable to use water as a solvent of a polymerization solution, and it is also preferable to use an inexpensive supporting electrolyte that is dissolved in water.
Among the supporting electrolytes that have been known so far, borodisalicylic acid and/or its salt are comparatively inexpensive and economically advantageous, but on the other hand, it is known that precipitation is rapidly produced when borodisalicylic acid and/or its salt are dissolved in water and stirred, or that even if it is not stirred, precipitation is gradually produced. Therefore, if borodisalicylic acid and/or its salt are selected as a supporting electrolyte for electrolytic polymerization of monomer with a π-conjugated double bond, an organic solvent has been selected. For example, in Patent Document 6 (JP 2-58818 A), a solid electrolytic capacitor is obtained by electrolytic polymerization in which a polymerization solution containing an organic solvent, a monomer such as pyrrol, and an alkyl-substituted ammonium borodisalicylate as a supporting electrolyte is used. The alkyl-substituted ammonium borodisalicylate is selected as the supporting electrolyte dissolved in the organic solvent. This document also states that a polymerized film with a problem in terms of thermal stability is obtained from a polymerization solution that is prepared by using ammonium borodisalicylate, which has low solubility in an organic solvent, as a supporting electrolyte, and adding water to organic solvents